(1) Field of the invention
This invention relates to a fiber reinforced plastics rod having helical ribs on their surfaces and a method for producing the same. The fiber reinforced plastics rods of the present invention can be used in place of the reinforcing steel rods, which are embedded in various concrete structures.
(2) Description of the Prior Art
In the conventional art, fiber reinforced plastics rods having helical elevated portions on their surfaces were made by the following steps.
In the first place, strands are made by arranging together 50 to 30,000 of long filaments of glass fiber, carbon fiber or synthetic fiber, and 8 to 120 of the strands are put together to obtain rovings or tows. They are then arranged together, and passed through and impregnated with a liquid of synthetic resin such as unsaturated polyester resin or epoxy resin to obtain impregnated materials. They are further passed through a die or scraped with a doctor knife in order to remove the excess resin clinging on them. After that, they are passed through a heated die or an oven so as to cure the impregnated resin, thereby obtaining rods having smooth surfaces. In the next place, helical grooves are formed in the surfaces of the rods using a lathe to leave helical elevated portions on the surfaces of the rods.
In these rods having helical elevated portions, however, because long fibers are embedded in the resin in parallel with the length of the rods, the long fibers in the surface portions are cut off in the process to form the helical grooves. As a result, the reinforcing fibers in the helical elevated portions (corresponding to screw threads) exist as short fibers oriented in a single direction. Accordingly, the strength in the helical elevated portions is lowered and the elevated portions are sometimes caused to split and scaled off from the body of plastics rods.
In order to avoid the disadvantage like this, an improved idea is disclosed in Japanese Laid-Open Patent Publication No. 61-235115. In the art disclosed in this reference, compression molding is carried out by introducing a synthetic resin material containing continuous reinforcing fiber into a compression mold to form a reinforced material having projections on the surface thereof. The compression mold is then opened and, simultaneously with the taking out of the formed reinforced material, the next uncured material subsequent to the already formed part is shifted into the compression mold and, by closing the mold, the compression molding is repeated again to form the next continued portion of the reinforced material having projections.
It is to be noted, however, that this method cannot be carried out continuously to produce the reinforced materials, which is not desirable in that the efficiency of molding process is not high.
Meanwhile, disclosed in Japanese Laid-Open Patent Publication No. 63-139735 is another kind of fiber reinforced resin rods. In the production of this fiber reinforced resin rod, an already cured fiber reinforced resin substrate rod is prepared by using a reinforcing material of fibers which are arranged in parallel to each other and oriented in the longitudinal direction of the rod. Continuous fiber is impregnated with uncured resin and it is applied helically to the surface of the above cured resin substrate rod and the applied impregnated fiber is then cured to obtained the final product of a fiber reinforced resin rod.
However, this rod is produced by helically applying continuous fibers which are impregnated with uncured resin to the surface of an already cured fiber reinforced substrate rod, and the curing operation is done again to harden the externally applied resin impregnated fiber. Accordingly, a boundary exists between the formerly cured substrate rod and the subsequently cured resin impregnated fibers. This means that both the materials are connected simply by physical adherent force. Therefore, when any external force that is larger than the force of adherence is applied to the boundary portion by some causes, both the materials are easily broken apart at the boundary portion.